Philip Jones Griffiths lives in a scruffy basement flat on the Goldhawk Road. It's not the sort of place where you would expect to find one of the world's most respected photographers living out his final years. A fashion photographer of equivalent stature would be found in one of the mansions that line the more salubrious Holland Park Avenue on the other side of Shepherd's Bush roundabout. But you don't take up a career as a war photographer for the money. Nor can you rightfully expect to survive as long as he has - he turns 70 this year.

So if you won't grow rich and can expect to die young, what drives a war photographer? "I come from a small village in Wales. It was highly religious and even though I'm a militant atheist the idea that you have to do something worthwhile in your life was implanted in my mind when I was very young," he says.

Don McCullin, a contemporary who covered the Vietnam War alongside him, has been quoted as saying that "not a single one of my photos has made the slightest bit of difference." But Griffiths could lay claim otherwise. His seminal book Vietnam Inc, first published in 1971, has been credited as one of the major factors in turning American opinion on the war. This degree of impact is rare and he dismisses as "naïve" the notion that photojournalists can single-handedly stop atrocities. But he does believe in the power of an image as a catalyst for changing opinion. "All I can tell you is that a month ago a guy came up to me and said (in a Welsh accent): 'I'm so grateful to you. I joined the army and a month later I came across Vietnam Inc. So I resigned straightaway.' Those are the moments which one is quite proud of."

Born in Rhuddlan in North Wales in 1936 he was based for a long time in New York but bowel cancer has necessitated his return to London for treatment. As a child he discovered the fun that could be had making explosions with chemistry sets and he soon progressed to developing photographs in the family bathroom. His parents misinterpreted his enthusiasms and packed him off to study chemistry in Liverpool. He later worked as night manager at Boots in London's Piccadilly, a job that provided him with the opportunity to photograph the prostitutes and drug takers who frequented the store in the middle of the night. Soon he was working for The Observer but later went freelance and was invited to join the renowned photographic agency Magnum.

His upbringing in a Welsh village appears to have had an indelible influence on his life, but perhaps in ways that his parents might not have expected. While he rejected the strict Wesleyan Methodism forced upon him in his youth, photography replaced it as his religion. He has been quoted as saying "I didn't choose photography. Photography chose me" and he sees his decision to document the Vietnam War as almost fateful. "It was inconceivable to me that I could be killed," he says, "although common sense would tell you that of course you could be killed…but it would have seemed particularly unfortunate had the bullet got me instead of the other guy."

This sense of destiny, coupled with a strong dose of common sense, seemed to keep at bay any fears he may have had on the battlefield. He claims he was never scared: "the longer you did it, the more you understood, the more questions you could ask the officers - where we were going, what we were doing, had the roads been cleared of mines? You were certainly in a far more exalted position than the average GI. You constantly had to make decisions and I'm certainly only here because on one or two occasions I made the right decision."

He describes the average war photographer as someone who likes adventure. "I've seen people who when the bullets start flying they don't walk - they dance like Nureyev and their eyes are glazed." So does he consider himself an adrenaline junkie like some of his colleagues? "Absolutely not. There were occasions when the guy in front of me was killed and you're not human if you don't get a bit of 'fright and flight.' But I just felt I had a purpose for being there."

He has often commented on his affinity with the Vietnamese. He identifies with the oppressed: his childhood spent growing up under the shadow of industrial England across the border allowed him to draw parallels with America's intervention in Vietnam. He recalls an incident in his childhood during World War 2 when he was offered a Mars bar by an American soldier from the nearby military base, a gesture he saw as a cynical PR exercise in winning over the minds of the locals. When he saw the same thing occurring in Vietnam he began to understand how the American invasion of Vietnam was not just military but cultural too. This cultural intrusion was to become one of the main themes of Vietnam Inc.

His life's work has been dominated by South East Asia. His small office / living room is lined with books on Vietnam. "There's hardly a day goes by when I don't buy another book on it," he says. He has also published two other books of photographs from his years there: Agent Orange: Collateral Damage in Viet Nam, which catalogues the horrific mutations inflicted on the local population by the US policy of spraying a herbicide to kill crops, and most recently Viet Nam at Peace, in which he revisited the country to observe its progress after thirty years of peace. But he has also worked in 140 other countries: he was in Cambodia ("much more dangerous than Vietnam"), Grenada ("not really dangerous"), the first Gulf War ("not dangerous at all") and Northern Ireland ("not dangerous but interesting because there was always that interface between the military and civilians which makes for meaningful photographs").

Life may have since slowed but he doesn't get bored. He expresses no desire to be back on the road photographing the latest conflict zone. "My experiences have taught me that you can either spread yourself wide and thin or you can concentrate and go deeper. For one reason or another I found Vietnam and Vietnam found me." Instead he has been busy reinforcing his reputation as the photographer who Henri Cartier-Bresson described as the best portrayer of war since Goya. And he has managed to spend more time with his family. He doesn't believe in marriage but has two daughters by two mothers.

He speaks candidly and lucidly about his life. Despite the sessions of chemotherapy that muddle his brain he keeps himself busy, giving talks at London's major photographic establishments and contributing to documentaries. Sound bites trip easily off his tongue in his now eroded Welsh accent, and while the same expressions crop up repeatedly in interviews, the repetitions reveal someone who has reflected long and hard on a life well lived and who in the latter part of his life is making sure he consolidates his position as one of the 20th century's most notable and influential photojournalists.